Optical inspection is commonly used in semiconductor device manufacturing to detect defects on the surface of a wafer, such as contaminant particles, scratches and unremoved portions of material layers. Defects can cause device failures, thus substantially reducing the process yield. Therefore, careful inspection is required to verify the cleanliness and quality both of unpatterned wafers and of patterned wafers at various stages in the manufacturing process.
A common method for inspecting semiconductor wafers is to scan a laser beam over the wafer surface, and measure the light scattered from each point on which the beam is incident. One such method, based on dark-field scattering detection, is proposed by Smilansky et al., in U.S. Pat. No. 6,366,690, whose disclosure is incorporated herein by reference. Smilansky et al. describe a wafer inspection system based on an optical detection head that comprises a laser and a number of light sensors, which are fed by fiberoptic light collectors arrayed around the laser. The optical head is positioned over the wafer surface, and the wafer is rotated and translated so that the laser beam scans over the surface. The sensors detect the radiation that is scattered from the surface in different angular directions simultaneously, as determined by the positions of the fiberoptics. The entire wafer surface is thus scanned, one pixel at a time, along a spiral path.
Another dark-field wafer inspection system is described by Marxer et al., in U.S. Pat. No. 6,271,916, whose disclosure is incorporated herein by reference. In this system, a laser beam is directed toward the wafer surface in a normal direction and scans the surface along a spiral path. An ellipsoidal mirror is used to collect the laser radiation that is scattered from the surface at angles away from the normal. Preferably, light scattered within a first range of angles is collected by one detector, while that scattered within a second range of angles is scattered by another detector. The different detector signals are used to distinguish large defects from small defects.
A further defect detection system based on this approach is described by Vaez-Iravani et al., in U.S. Pat. No. 6,538,730, which is also incorporated herein by reference. In this case, different wide- and narrow-angle collection channels are used. Signals obtained from the narrow and wide collection channels may be compared to distinguish between micro-scratches and particles. Forward-scattered radiation may also be collected and used for this purpose. The intensity of scattering may further be measured using sequential illumination with S- and P-polarized radiation.
Chuang et al. describe an imaging system with high numerical aperture (NA) in U.S. Pat. No. 6,392,793, whose disclosure is incorporated herein by reference. The system is based on a catadioptric group of mirrors and lenses, which can be used to collect reflected, diffracted, and scattered light over a range of angles. The system has several applications, including dark-field imaging.
Kinney et al. describe an optical inspection module and method for detecting particles and defects in U.S. Pat. No. 5,909,276, whose disclosure is incorporated herein by reference. The module includes a light source, which illuminates a surface under inspection at a grazing angle of incidence. A lens is oriented to collect non-specularly reflected light scattered from the light beam path by defects on the surface. A photodetector array in the focal plane of the lens receives the scattered light. Each pixel of the array corresponds to an area on the surface, and the plurality of pixels together form a field of view that covers substantially the entire surface.
Speckle is a well-known effect in imaging systems that use coherent illumination, due to the strong autocorrelation of the beam amplitude. In coherent illumination systems known in the art, which are typically based on continuous wave (CW) laser illumination, the laser beam is passed through a rotating diffuser, which reduces the autocorrelation and thus reduces the speckle contrast accordingly. Alternatively, the laser beam may be passed through a bundle of optical fibers of different lengths, as described, for example, by Suganuma in U.S. Pat. No. 6,249,381, whose disclosure is incorporated herein by reference. Enhanced de-speckling may be achieved by using two optical fiber bundles disposed sequentially along the light path, as described by Karpol et al., in U.S. Patent Application Publication US 2002/0067478 A1, which is assigned to the assignee of the present patent application and whose disclosure is incorporated herein by reference.